1. Field of the Invention
This invention relates to multilayered packaging materials being excellent in food keeping performance due to their high gas barrier properties, particularly to multilayered packaging films, in particular lids to be used for retorting or pauches to be used for retorting, which are unparalleled with respect to excellent transparency together with high gas barrier properties. This invention also includes cup-shape or tray-shape containers to be used for retorting having high gas barrier properties.
2. Description of the Prior Art
Laminate films consisting of aluminum foil and polyolefin, particularly polypropylene, and metal lids for double-seaming are used as lids to be used for retorting. Both of them have nearly perfect gas barrier properties, while they have a drawback of not being transparent, thereby rendering the contents invisible. The contents should necessarily be seen from outside in view of appeal to consumers, as well as of the requirement for the contents not to be spilled when the container is opened, or of convenience when the lid is locally pierced with a hole before the container is heated in the micrwave or the like. However, transparent lids made of plastic film are currently only in use for limited uses due to their insufficient gas barrier properties, particularly insufficient oxygen gas barrier properties. Polyvinylidene chloride polymer (principally vinylidene chloride-vinyl chloride copolymers, hereinafter sometimes referred to as PVDC) is a representative example of gas-barrier resins currently in use for lids, and is used in a form of laminated film obtained by laminating thereon a biaxially oriented nylon film (hereinafter sometimes referred to as ON) for the outer layer and a polypropylene film (hereinafter sometimes referred to as PP) for the inner layer which is a sealing layer by dry laminating or the like process. The oxygen gas transmission rate or oxygen permeability (hereinafter abbreviated as OTR) of PVDC is 3 to 20 cc/m.sup.2.multidot. day.multidot.atm per 20 .mu.. Since an OTR of about 0.8 cc/m.sup.2.multidot. day.multidot.atm is required for a packaging material to keep food therein in a good condition for 6 months under normal circumstances, PVDC must have a thickness of at least 75 .mu.. Then, if PVDC film is 75 .mu. thick, it will have a high haze and become yellowish. The film thickness of PVDC therefore is generally 15 to 30 .mu., and about 50 .mu. only for some uses where a high barrier property is particularly required. Thus, PVDC lids cannot be said to have sufficient food keeping performances.
Recently, films made of vinylidene chloride-ethyl acrylate copolymers are known to have particularly high gas barrier properties among PVDC-related polymers. The OTR is 1.2 cc.multidot.20 .mu./m.sup.2.multidot. day.multidot.atm. The polymer, however, has drawbacks of becoming opaque and increasing its OTR by retort treatment, thereby being unuseable as a transparent lid for retorting.
Among gas-barrier resins other than PVDC, there are metaxylylenediamine-adipic acid condensate (MX-nylon), polyamide, PVDC-coated polyamide and the like being used for lids. They are all of low oxygen gas barrier properties, and hence are used only for such limited kinds of foods as have low oxidation-degradation liability or are transported and stored at low temperatures.
Pauches among packaging films for retorting are under still severer conditions. That is, while a lid is only a part of a container, transparent packaging films used for pauches, skin-packs, deep-drawn packages, rocket packaging and the like constitute the whole container. Since any transparent packaging films used currently for the above packagings are not of sufficient gas barrier properties, the packages after retort treatment are all transported and stored only at low temperatures.
Ethylene-vinyl alcohol copolymer resin (hereinafter sometimes referred to as EVOH) is a thermoplastic resin having the highest gas barrier properties of today and ones exhibiting OTR's of from 0.3 (at 60% RH) to 0.75 (80% RH) cc.20 .mu.m.sup.2.multidot. day.multidot.atm are used as general-purpose brands (EVAL.RTM.Type F, made by Kuraray Co.). EVOH is widely used as a gas-barrier material for retortable containers such as cups and trays, for which employed are multilayered structures comprising EVOH for the intermediate layer and PP which has low moisture permeability as the inner and outer layers. For the purpose of further increasing food keeping performance, there is a method which comprises incorporating a drying agent in an adhesive resin layer between PP layer and EVOH layer (U.S. Pat. No. 4,407,897) or a method which comprises incorporating a drying agent in EVOH layer itself (U.S. Pat. No. 4,425,410).
Further, Japanese Patent Publication No. 24254/1986 (B.P. 2,006,108) discloses a plastic bottle for containing food, consisting of an outer layer of a polyamide, an intermediate layer of a gas-barrier resin (e.g. EVOH) and an inner layer of a polyolefin or thermoplastic polyester; and also describes that as the gas-barrier resin a blend of EVOH with 6/66 copolymer nylon in a blending ratio of 50:50 by weight. However, it has been found that if the blend of EVOH containing polyamide (nylon) in as much as 50 wt % is used, there will occur problems of not only a decrease in gas barrier property but also whitening when stored after retort treatment of the multilayered package. This fact is apparent from Comparative Example 6 later described herein.
Japanese Patent Publication No. 38103/1954 (B.P. 1,545,096) describes a container comprising inner and outer layers of a moisture-resistant resin or creep-resistant resin having a low moisture permeability and an intermediate layer of a composition incorporating blended therein a gas-barrier resin (e.g. EVOH/polyamide in a ratio of from 90:10 to 10:90), but it does not describe about providing an outer layer of a resin having a comparatively high moisture permeability and an inner layer of another resin having a low moisture permeability, or further about using for the intermediate layer therefor a layer comprising EVOH blended with a relatively small amount of a polyamide. This patent does not describe either about a packaging material comprising an outer layer of the blended gas-barrier resin composition and an inner layer of a resin having a low moisture permeability, or about the use of such packaging material for packaging materials for retort sterilization or boil sterilization.
Japanese Patent Laid Open No. 54-78749 describes a blend of EVOH and polyamide and said blend being laminatable with other thermoplastic resin, but it does not describe a package material comprising an outer layer of said blend and an inner layer of a resin having a low moisture permeability.